Sulfuric acid is used in large quantities in various industries at grades (purity and concentration) required for these industries. In general, wasted sulfuric acid is subjected to the following processes:    (1) In situ treatment of wastewater (neutralization) generated in factories;    (2) Treatment of wastewater (neutralization) in recycling firms;    (3) Reuse as a neutralizer in factories;    (4) Recycling (distillation purification) in factories or in recycling firms; and    (5) Treatment for reuse as a raw material of alum (or aluminum sulfate) in specialized firms.
Among them, case (1) is most common. In this case, a large amount of alkaline chemical (calcium hydroxide) and various wastewater treatment agents (e.g. an inorganic flocculant, a polymer flocculant, and a hydrogen peroxide decomposition accelerator) are required. Thus, large amounts of sludge and wastewater are generated. This is undesirable from the viewpoints of environmental and resource aspects. Case (2) is similar to case (1) although the waste sulfuric acid is treated outside factories. Since large amounts of sludge and wastewater are also generated, case (2) is undesirable from the viewpoints of environmental and resource aspects. In case (3), since the demand for waste sulfuric acid as a neutralizer is low, the surplus waste sulfuric acid must be treated as in case (1) or (2). Consequently, this is not satisfactory. Case (4) requires a large-scale plant and energy for recycling waste sulfuric acid and causes many industrial problems such as energy, cost, site space, safety, and handling. Therefore, case (4) is rarely used. In case (5), since waste sulfuric acid is seldom treated as a valuable resource, typically, recycling firms charge for services or buy it at very low prices (at most a few yen/Kg). In this case, the usage rate of waste sulfuric acid is usually equal to or less than 10 mass percent. This rate should be increased.
Since the added value of waste sulfuric acid in the above-mentioned process is low, conventionally, waste sulfuric acid is generally recycled in factories using sulfuric acid or recycling firms with recycling charges. For example, waste sulfuric acid generated in semiconductor plants usually contains several tenths of a percent to several percent of hydrogen peroxide. Therefore, the waste sulfuric acid is treated (neutralization) or is used as a neutralizer in these plants. In some cases, the waste sulfuric acid is used at sites other than the plants as a raw material for aluminum sulfate. Even if the waste sulfuric acid is recycled, the use is limited to cascade utilization (simply as an alternate of industrial-grade sulfuric acid) that has a significantly lower added value. As a result, a method and an apparatus for effectively recycling waste sulfuric acid in any manufacturing factory or firm as a raw material having a high added value are awaited.
It is an object of the present invention to provide a recycling apparatus for changing waste sulfuric acid into recycled sulfuric acid having a high added value in an industrially profitable manner.